We don't know what it is, we don't know of its origins, and we're not sure where to look for it. Why is finding dark matter still so important?
The first thing that goes wrong whenever a new discovery is reported, an old one is invalidated, or some vaguely important scientific result is announced has often to do with misrepresentation in the mainstream media. Right now, we’re in the aftermath of one such event: the October 30 announcement of results from a very sensitive dark matter detector. The detector, called the Large Underground Xenon Experiment (LUX), is installed in the Black Hills of South Dakota and operated by the Sanford Underground Research Facility.
Often the case is that what gets scientists excited may not get the layman excited, too, unless the media wants it to. So also with the announcement of results from LUX:
• The detector hasn’t found dark matter
• It hasn’t found a particular particle that some scientists thought could be dark matter in a particular energy range
• It hasn’t ruled out that some other particles could be dark matter.
Unfortunately, as Matt Strassler noted, the BBC gave its report on the announcement a very misleading headline. We’re nowhere near figuring out what dark matter is as much as we’re figuring out what dark matter isn’t. Both these aspects are important because once we know dark matter isn’t something, we can fix our theories and start looking for something else. As for what dark matter is… here goes.
What is dark matter?
Dark matter is a kind of matter that is thought to occupy a little more than 80 per cent of this universe.
Why is it called ‘dark matter’?
This kind of matter’s name has to do with a property that scientists believe it should have: it does not absorb or emit light, remaining (optically) dark to our search for it.
What is dark matter made of?
We don’t know. Scientists think it could be composed of strange particles. Some other scientists think it could be composed of known particles that are for some reason behaving differently. At the moment, the leading candidate is a particle called the WIMP (weakly interacting massive particle), just like particles called electrons are an indicator of there being an electric field or particles called Higgs bosons are an indicator of there being a Higgs field. A WIMP gets its name because it doesn't interact with other matter particles except through the gravitational force.
We don’t know how heavy or light WIMPs are or even what each WIMP’s mass could be. So, using different detectors, scientists are combing through different mass-ranges. And by 'combing', what they're doing is using extremely sensitive instruments hidden thousands of feet under rocky terrain (or obiting the planet in a satellite) in an environment so clean that even undesired particles cannot interact with the detector (to some extent). In this state, the detector remains on 'full alert' to note the faintest interactions its components have with certain particles in the atmosphere - such as WIMPs.
The LUX detector team, in its October 30 announcement, ruled out that WIMPs existed in the ~10 GeV/c2 mass range (because of a silence of its components trying to pick up some particles in that range). This is important because results from some other detectors around the world suggested that a WIMP could be found in this range.
(GeV/c2 is a measure of energy and, by the mass-energy equivalance, of mass, too. To compare: A proton weighs about 1 GeV/c2 in its non-accelerated state.)
Can we trust LUX’s result?
Pretty much but not entirely - like the case with most measurements in particle physics experiments. Physicists announcing these results are only saying they aren’t likely to be any other entities masquerading as what they’re looking for. It’s a chance, and never really 100 per cent. But you’ve got to draw the line at some point. Even if there’s always going to be a 0.000...01 per cent chance of something happening, the quantity of observations and the quality of the detector should give you an idea about when to move on.
Where are the other detectors looking for dark matter?
Some are in orbit, some are underground. Check out FermiLAT, Alpha Magnetic Spectrometer, Payload for Antimatter Exploration and Light-nuclei Astrophysics, XENON100, CDMS, Large Hadron Collider, CoGeNT, etc.
So how was BBC wrong with its headline?
We’re not nearing the final phase of the search for dark matter. We’re only starting to consider the possibility that WIMPs might not be the dark matter particle candidates we should be looking for. Time to look at other candidates like axions. Of course, it wasn’t just BBC. CBS and Popular Science got it wrong, too, together with a sprinkling of other news websites.
Why do we need dark matter?
We haven’t been able to directly detect it, we think it has certain (unverified) properties to explain why it evades detection, we don’t know what it’s made of, and we don’t really know where to look if we think we know what it’s made of. Why then do we still cling to the idea of there being dark matter in the universe, that too in amounts overwhelming ‘normal’ matter by almost five times?
Answer: Because it’s the simplest explanation we can come up with to explain certain anomalous phenomena that existing theories of physics can’t.
When the universe was created in a Big Bang, matter was released into it and sound waves propagated through it as ripples. The early universe was very, very hot, and electrons hadn’t yet condensed and become bound with the matter. They freely scattered radiation, whose intensity was also affected by the sound waves around it.
About 380,000 years after the Bang, the universe cooled and electrons became bound to matter. After this event, some radiation pervading throughout the universe was left behind like residue, observable to this day. When scientists used their knowledge of these events and their properties to work backwards to the time of the Bang, they found that the amount of matter that should’ve carried all that sound didn’t match up with what we could account for today.
They attributed the rest to what they called dark matter.
Another way this mass deficiency manifests is in the observation of gravitational lensing. When light from a distant object passes near a massive object, such as a galaxy or a cluster of galaxies, their gravitational pull bends the light around them. When this bent beam reaches an observer on Earth, the image it carries will appear larger because it will have undergone angular magnification. If these clusters didn’t contain dark matter, physicists would observer much weaker lensing than they actually do.
That’s not all. The stars in a galaxy rotate around the galactic centre, where most of its mass is located. According to theory, the velocity of the stars in a galaxy should drop off the farther they get from the centre. However, observations have revealed that, instead of dropping off, the velocity is actually almost constant even as one gets farther from the centre. This leaves the outermost stars rotating about the galactic centre at velocities that should 'fling' them away from the galaxy. Evidently that isn't the case. So, something is also pulling the outermost stars inward, holding them together and keeping them from flying outward and away. The incredible prowess of this inward force astrophysicists think could be due to the gravitational force of dark matter.
So… what next?
LUX was a very high sensitivity dark matter detector, the most sensitive in existence actually. However, its sensitivity is attuned to look for low-mass WIMPs, and its first results rule out anything in the 5-20 GeV/c2 range. WIMPs of a higher mass are still a possibility, and, who knows, might be found at detectors that work with the CERN collider.
Moreover, agreement between various detectors about the mass of WIMPs has also been iffy. For example, detectors like CDMS and CoGeNT have hinted that a ~10 GeV/c2 WIMP should exist. LUX has only now ruled this out; the XENON100 detector, on the other hand, has been around since 2008 and has been unable to find WIMPs in this mass-range altogether, and it’s more sensitive than CDMS or CoGeNT.
What’s next is some waiting and letting the LUX carry on with its surveys. In fact, the LUX has its peak sensitivity at 33 GeV/c2. Maybe there's something there. Another thing to keep in mind is that we’ve only just started looking for dark matter particles. Remember how long it took us to figure out ‘normal’ matter particles? Perhaps future higher sensitive detectors (like XENON1T and LUX-ZEPLIN) have something for us.